全文获取类型
收费全文 | 96463篇 |
免费 | 1198篇 |
国内免费 | 1743篇 |
专业分类
测绘学 | 3129篇 |
大气科学 | 6943篇 |
地球物理 | 18094篇 |
地质学 | 37273篇 |
海洋学 | 7963篇 |
天文学 | 18998篇 |
综合类 | 2364篇 |
自然地理 | 4640篇 |
出版年
2022年 | 543篇 |
2021年 | 903篇 |
2020年 | 997篇 |
2019年 | 1047篇 |
2018年 | 6679篇 |
2017年 | 5867篇 |
2016年 | 5004篇 |
2015年 | 1586篇 |
2014年 | 2501篇 |
2013年 | 4035篇 |
2012年 | 3478篇 |
2011年 | 5813篇 |
2010年 | 4783篇 |
2009年 | 5830篇 |
2008年 | 4945篇 |
2007年 | 5446篇 |
2006年 | 3110篇 |
2005年 | 2336篇 |
2004年 | 2546篇 |
2003年 | 2402篇 |
2002年 | 2205篇 |
2001年 | 1796篇 |
2000年 | 1695篇 |
1999年 | 1327篇 |
1998年 | 1415篇 |
1997年 | 1285篇 |
1996年 | 1112篇 |
1995年 | 1041篇 |
1994年 | 975篇 |
1993年 | 832篇 |
1992年 | 812篇 |
1991年 | 813篇 |
1990年 | 868篇 |
1989年 | 673篇 |
1988年 | 675篇 |
1987年 | 741篇 |
1986年 | 642篇 |
1985年 | 862篇 |
1984年 | 898篇 |
1983年 | 797篇 |
1982年 | 779篇 |
1981年 | 672篇 |
1980年 | 681篇 |
1979年 | 632篇 |
1978年 | 629篇 |
1977年 | 503篇 |
1976年 | 493篇 |
1975年 | 500篇 |
1974年 | 448篇 |
1973年 | 481篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
Expression of putative zinc-finger protein lcn61 gene in lymphocystis disease virus China (LCDV-cn) genome 总被引:1,自引:0,他引:1
An open reading frame (lcn61) of lymphocystis disease virus China (LCDV-cn), probably responsible for encoding putative zinc-finger proteins was amplified
and inserted into pET24a (+) vector. Then it expressed in E. coli BL21 (DE3), and His-tag fusion protein of high yield was obtained. It was found that the fusion protein existed in E. coli mainly as inclusion bodies. The bioinformatics analysis indicates that LCN61 is C2H2 type zinc-finger protein containing
four C2H2 zinc-finger motifs. This work provides a theory for functional research of lcn61 gene.
Supported by High Technology Research and Development Program of China (863 Program, No. 2006AA100309) 相似文献
122.
S. S. Komissarov 《Astrophysics and Space Science》1990,165(2):313-323
A fluid model of the jets in Fanaroff-Riley I class, for extended extragalactic radio sources, based on the idea that they are turbulent low Mach number supersonic flows consisting of relativistic and nonrelativistic plasma, has been developed. Mean flow approach and modified mixing length theory by Prandtl are used to obtain a corresponding mathematical tool. It is assumed that magnetic field is weak. Boundary conditions concerned the problem of the jet propagation through a non-uniform atmospheres of an elliptical galaxies are considered. Short discussion of the observational data determining the choice of the input parameters of the jets is given. 相似文献
123.
Summary The summer monsoon circulation shows various spatial and temporal oscillations and often a combination of systems produces
an integrated effect. In this study phases of the southwest (SW) monsoon have been identified in an objective manner with
the help of T-mode principal component analysis (PCA) of weekly rainfall anomalies. Mean composite charts have been prepared
utilising all available upper air data (1977–1986) for each category of the SW monsoon epochs identified by the PCA. These
sets of charts have been constructed for both the strong and weak phases associated with the first four significant principal
components (PCs).
A well defined east-west oriented trough system, extending from about 28° N Latitude/65° E Longitude to 20° N Latitude/90° E
Longitude, in the lower levels, has been the main feature associated with the strong phase of the monsoon corresponding to
PC I. The trough in the lower levels is more marked in the eastern half compared to the western half in both the sets of charts
associated with strong phases of the monsoon related to the PC II and PC III. With PC II, the position of the troughs in the
lower levels is further north of its location in PC III. The east-west trough system associated with the strong phase of PC
IV has a large southward tilt with height. The charts corresponding to the weak phases of these PCs have synoptic features,
such as the position of the trough close to the foothills of the Himalayas, and the shifting of middle and upper tropospheric
anticyclones to the south.
The study suggests an objective method of interpretation of principal components by utilising synoptic data. In addition,
synoptic models and data sets corresponding to different phases of the monsoon can also be prepared in an objective manner
by such PCA.
Received July 18, 1997 Revised April 30, 1999 相似文献
124.
A. Kereszturi Z. Blumberger S. Józsa Z. May A. Müller M. Szabó M. Tóth 《Meteoritics & planetary science》2014,49(8):1350-1364
Analysis of the NWA 2086 CV3 chondrite showed a matrix/chondrule ratio of 52%, similar to Bali, Mokoia, and Grosanaja. Nearly twice as many chondrule fragments as intact ones demonstrate that an early fragmentation phase occurred prior to final accretion. After this event, no substantial mechanical change or redeposition is evident. Rims with double‐layered structures were identified around some chondrules, which, in at least one case, is attributed to an accretionary origin. The rim's outer parts with a diffuse appearance were formed by in situ chemical alteration. During this later process, Mg content decreased, Fe content increased, and olivine composition was homogenized, producing a rim composition close to that of the matrix. This alteration occasionally happened along fractures and at confined locations, and was probably produced by fluid interactions. Iron oxides are the best candidate for a small grain‐sized alteration product; however, technical limitations in the available equipment did not allow exact phase identification. These results suggest that NWA 2086 came from a location (possible more deeply buried) in the CV parent body than Mokoia or Bali, and suffered less impact effects—although there is no evidence of sustained thermal alteration. This meteorite may represent a sample of the CV parent asteroid interior and provide a useful basis for comparison with other CV meteorites in the future. 相似文献
125.
126.
The study of trace metal speciation has benefited from a great deal of interest leading to the development and the diversification of sequential extraction schemes (SES), which triggered the need for harmonization by the standard, measurements and testing programme (SM & T). However, some uncertainties still persist in the application of the harmonized 3-step protocol, because of the difficulty in conceptualizing a technique that can be applied to environmental samples irrespective of their nature. The alternative method proposed in this study is based on the solubility of the sample components progressively dissolved during the course of an acidimetric titration by 1 mol L−1 HCl. The major HCl-reactive mineralogical components are identified using mass balance calculation of H+ consumed by their dissolution together with the amount of major elements released into solution. The speciation of minor and trace elements is investigated by comparing their titrations to those of the major elements. This approach is much simpler than SES because it uses simple non-selective H+ at room temperature instead of a complex experimental design of so-called specific reagents. The different mineral components of the solid are no longer operationally defined and the problem of selectivity is irrelevant to the titration approach. The method was applied to several sediment samples from the Scheldt estuary and the particulate phase was further examined by Scanning Electron Microprobe and X-ray Diffraction techniques. The nearly complete consumption of H+ in the suspension is balanced by the total dissolution of carbonates and Fe-oxyhydroxides. In contrast to the speciation inferred from the Tessier SES, the acidimetric titration has demonstrated that the carbonate phase does not significantly contain trace metals with the exceptions of 40% of the Mn and 30% of the Co. In contrast, the Fe-oxyhydroxides seem to play a major role and account for 70% of Pb and 20% of Cr, in addition to 60% of P and additional amounts of 20% Co and 40% Mn. 70% of the Cu also occur in the oxyhydroxide phase, more likely coprecipitated with gibbsite. 90% of the Cd and 85% of Zn can be attributed to the Acid Volatile Sulfide (AVS) phase as evidenced from S titration. The acidimetric titration method not only provides information on the speciation of trace metals but also allows the quantification of their reactivity and mobility, if one considers that the titration roughly mimics pH changes that may occur as a result of chemical disequilibrium in the environment. The results demonstrate the potential of the acidimetric titration as an alternative to SES protocols in geochemical and environmental regulation studies. This method is applicable to a wide variety of environmental materials with little or minor adjustments. 相似文献
127.
The intellection of seismic wave propagation in coal measures demands direct observation of the wavefield progression. Two vertical seismic profiles with high spatial and temporal sampling, were recently recorded in the Sydney Basin coalfields as part of an experimental coal seismic program. Static corrections and interval velocities were obtained by an automated system to determine first kicks and pulse rise times. Upgoing and downgoing waves were separated in the f—k-plane using a novel technique of contour slice filtering. The isolated upgoing waves clearly display reflections from the major coal seams within the stratigraphic sequence. The downgoing wave spectra were subjected to attenuation analysis. The deduced specific quality factor Q for Permian coal measure rocks lies in the range 20–70. Similar estimates were obtained in the time domain from measurements of pulse broadening. Synthetic VSP seismograms, computed using an exact recursive formulation, are an indispensable aid to interpretation. They illustrate the filtering effects of coal seams and sequences, and the effects of the contribution of internal and free-surface multiple reflections in the recorded wavetrains. 相似文献
128.
It is proposed that Saturn's electrostatic discharges (SED) might be generated in the planet's equatorial atmosphere, perhaps as lightning from a storm system. The 10h10m periodicity of the signal envelope duplicates that of Saturn's equatorial jet. The rings shield the atmosphere from solar EUV photons, and thereby substantially reduce the local ionospheric cutoff frequency to allow low-frequency SED to leak out. Many of the unusual properties of SED could be explained in terms of changes in the storm system, the relative spacecraft position in the beaming pattern of the source, local refraction of the signal by the highly disturbed ionosphere, and the influence of the ring particles on the highest frequency component of SED. A comparison of SED with planetary lightning on other planets shows that the two are similar in general character and some time behavior; the power output of SED may be higher than most planetary lightnings but that is unclear because of uncertainties in the measurements and variations in the signal's spectrum. Our simple discussion suggests that lightning could be a viable source for SED and that exotic ring mechanisms are not necessarily required. 相似文献
129.
Robert Chapman Robert S. Priddey Nial R. Tanvir 《Monthly notices of the Royal Astronomical Society》2009,395(3):1515-1522
There is increasing evidence of a local population of short duration gamma-ray bursts (sGRB), but it remains to be seen whether this is a separate population to higher redshift bursts. Here we choose plausible luminosity functions (LFs) for both neutron star binary mergers and giant flares from soft gamma repeaters (SGR), and combined with theoretical and observed Galactic intrinsic rates we examine whether a single progenitor model can reproduce both the overall Burst and Transient Source Experiment (BATSE) sGRB number counts and a local population, or whether a dual progenitor population is required. Though there are large uncertainties in the intrinsic rates, we find that at least a bimodal LF consisting of lower and higher luminosity populations is required to reproduce both the overall BATSE sGRB number counts and a local burst distribution. Furthermore, the best-fitting parameters of the lower luminosity population agree well with the known properties of SGR giant flares, and the predicted numbers are sufficient to account for previous estimates of the local sGRB population. 相似文献
130.
Kieran P. Donaghy 《Journal of Geographical Systems》2011,13(1):17-30
In the literatures of regional science, urban economics, and urban development planning, a working assumption is that individuals
respond to incentives and regulations, given their preferences. Models for planning and policy analyses are used to consider
what might occur if the incentives or regulations were different. In these models, however, preferences are usually assumed
to be given and stable, and agents are usually assumed to be homogeneous. This paper focuses on the implications of making
preferences in models of policy implementation endogenously determined and time varying heterogeneous agents. We consider
first the recent literature on intertemporal choice and preference change, which cuts across many disciplines, and more briefly
the literature on norm-regarding behavior. We then elaborate a simple model of transportation demand—from a static to a dynamic
orientation, from fixed and exogenously given preferences of strictly self-regarding agents to endogenously determined and
policy-induced preferences of heterogeneous agents—and illustrate its characteristics with simple numerical examples. 相似文献